Hearing loss may be due to many different causes, but is generally of two types: conductive and sensorineural. Of these, conductive hearing loss occurs where the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss may often be helped by use of conventional hearing aids, which amplify sound so that acoustic information, in the form of pressure waves, reaches the cochlea and the hair cells. Some types of conductive hearing loss are also amenable to alleviation by surgical procedures.
In many people who are profoundly deaf, however, the reason for their deafness is sensorineural hearing loss. This type of hearing loss is due to the absence or the destruction of the hair cells in the cochlea which are needed to convert acoustic signals into auditory nerve impulses. People with sensorineural hearing loss are unable to derive any benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because their mechanisms for converting sound energy into auditory nerve impulses have been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses are not generated directly from sounds.
To overcome sensorineural deafness, numerous Implantable Cochlear Stimulation (ICS) systems—or cochlear prosthesis—have been developed. Such systems seek to bypass the hair cells in the cochlea by presenting electrical stimulation to the auditory nerve fibers directly, leading to the perception of sound in the brain and at least a partial restoration of hearing function. The common denominators in most ICS systems have been the implantation of electrodes into the cochlea, and a suitable external source of an electrical signal for the electrodes.
An ICS system operates by direct electrical stimulation of the auditory nerve cells, bypassing the defective cochlear hair cells that normally convert acoustic energy into electrical activity in the nerve cells. In order to effectively stimulate the nerve cells, the electronic circuitry and the electrode array of the ICS system perform the function of separating the acoustic signal into a number of parallel channels of information, each representing the intensity of a narrow frequency band within the acoustic spectrum. Ideally, the electrode array would convey each channel of information selectively to the subset of auditory nerve cells that normally transmit signals within that frequency band to the brain. Those nerve cells are arranged in an orderly tonotopic sequence, from high frequencies at the basal end of the cochlear spiral to progressively lower frequencies towards the apex, and ideally the entire length of the cochlea would be stimulated to provide a full frequency range of hearing. In practice, this ideal is not achieved, because of the anatomy of the cochlea which decreases in diameter from the base to the apex, and exhibits variations between patients. Because of these difficulties, known electrodes can at best be promoted to the second turn of the cochlea.
The signal provided to the electrode array is generated by a signal processing element of the ICS system. In known ICS systems, the acoustic signal is processed by a family of parallel Bandpass (BP) filters, or the equivalent, resulting in M stimulation channels. Generally, the important information for speech understanding is contained in subset of all the M channels. This subset is usually made up of the channels containing the highest amplitude signals among the M channels at any given time. One common stimulation strategy selects N of the M channels for stimulation based on the amplitude of the signals in the channels. There are at least two advantages to the N out of M (N-of-M) strategy. First, an N-of-M strategy allows higher stimulation rates for a given pulse width when using non simultaneous stimulation. Second, an N-of-M strategy performs a data reduction function, in that BP channels that contain lower amplitude information are effectively muted, limiting their contribution to electrode interaction problems.
But, there are potential disadvantages to N-of-M strategies as well. The data reduction function of a standard N-of-M strategy is implemented using an all or nothing algorithm, selecting only the channels with the highest amplitude signals in a given stimulus frame. This means that all information in the lower amplitude channels is lost during that frame. This could be very disadvantageous in situations where the overall frequency distribution remains relatively constant for a period of time, such as when listening in certain noisy environments or detecting background sounds during vowels. One example of this would be someone honking a horn while someone is talking. If the horn is loud enough, its spectral content would overwhelm the talker, and the standard N-of-M decision matrix would only deliver envelope information to the pulse generator for those channels which contain “horn content”. All of the other channels would be effectively muted.
What is needed is a method for processing the channels of an ICS system, which method provides the advantages of an N-of-M stimulation strategy, without muting channels having low to moderate signal amplitudes.